In many locations there is a shortage of water such as in arid regions of the planet. In other locations there is a shortage of potable water such as in areas which have poor water treatment or areas which have experienced a natural disaster (eg. a flood) or a man made disaster (eg. a war). In many cases, however, the ambient air contains sufficient moisture that, if extracted, could provide a supply of water to these regions.
One method for extracting water from air is to compress the air to the point where water vapour condenses to form liquid water. This method requires large amounts of energy and equipment involving many moving parts including seals which must withstand high pressures. The cost and complexity of this method makes it undesirable.
Another method is disclosed in U.S. Pat. No. 4,726,817. Pursuant to this disclosure, the ambient air is canalized and cooled in a free space. The cooled air is then passed through a curtain of hygroscopic fibres where water vapour condenses into liquid water which is evacuated through a conduit. To date, no device to obtain water in useable form from the atmosphere has achieved commercial acceptance
In industry, it is sometimes necessary to remove water from air and different methods have been developed to achieve this result. For example, water may be removed from air by passing the air over a cool surface to condense out water. This technique is used in various areas of art such as to separate water from process flow streams in industry or to provide drier chilled air for climate control. U.S. Pat. No. 4,726,817 also used the concept of cooling the air to condense water vapour.
Industry has also used liquid desiccants for extracting water from air. For example, U.S. Pat. No. 4,189,848 discloses a process in which a liquid desiccant is used to dehumidify air for the purpose of drying a crop. In a closed loop portion of the process, air for drying, on leaving a drying bin, is contacted with a liquid desiccant to remove moisture from it, heated, and recirculated to the drying bin. The liquid desiccant is re-concentrated after contact with the air so that it may be re-used.
The effectiveness of liquid desiccants can be expressed in terms of their "drying efficiency" and "drying capacity". "Drying efficiency" is the ratio of total water exposed to the hygroscopic solution to the amount of water removed. "Drying capacity" is the quantity of water that a unit mass of desiccant can extract from the air.
The drying efficiency and drying capacity of a hygroscopic solution is in part dependant on the partial pressure of water vapour in the air and on the concentration of the solute, which effects the partial pressure of water vapour in the desiccant. Although other factors influence the reaction, a hygroscopic solution having a high concentration of solute, and thus a low partial pressure of water vapour, quickly adsorbs water from air having a higher partial pressure of water vapour and so its initial drying efficiency is high. As water is adsorbed in the hygroscopic solution, the partial pressure of water vapour in the solution increases and the rate of water adsorption slows down. Eventually, the hygroscopic solution and the air reach equilibrium and no more water will be adsorbed. In a regenerative process, the extracted water must therefore be separated from the hygroscopic solution to return it to its initial concentration. This regeneration step accounts for a significant amount of the energy required in a regenerative process.
The focus of the process disclosed in U.S. Pat. No. 4,189,848 is on reducing the amount of water in the relatively fixed volume of air that is recirculated to the drying bin. As the air is recirculated, increased amounts or water are removed from the air until the air reaches the required level of dryness. Any water extracted from the air is an unwanted by-product. Therefore, in designing the drying cycle to reach the required level of dryness, the drying efficiency of the liquid desiccant is a primary design criteria and the process is designed to favour the drying efficiency, and not the drying capacity, or the liquid desiccant.